Members used typically in automobiles are required to have not only high strength and high ductility (elongation) but also superior stretch flange formability from the viewpoints of ensuring collision safety and being environmentally friendly (increasing fuel efficiency). Specifically, demands have been made to provide high-strength steel sheets excellent both in balance between strength and elongation (hereinafter also referred to as “TS-EL balance” or “TS×EL”) and balance between strength and stretch flange formability (hereinafter also referred to as “TS-λ balance” or “TS×λ”). These balances are indices for formability.
Multi-phase structure steel sheets are known as high-strength steel sheets excellent in formability. The multi-phase structure steel sheets contain a ferrite matrix (main phase) and a second phase structure including austenitic low-temperature transformation phases such as martensite and bainite. The second phase structure can contain various components. Typically, Japanese Unexamined Patent Application Publication (JP-A) No. 2006-342373 discloses a high-tensile galvanized steel sheet containing martensite, bainite, retained austenite, or a mixture of them and excelling typically in strength-ductility balance; JP-A No. 2007-009317 discloses a high-strength cold-rolled steel sheet containing austenitic low-temperature transformation phases of martensite, bainite, and pearlite and excelling in stretch flange formability; JP-A No. 2003-193188 discloses a high-tensile galvannealed steel sheet mainly containing bainite or pearlite as a second phase structure; and JP-A No. 2004-211126 discloses a galvanized steel sheet containing not regular martensite structure but tempered martensite as a second phase structure and excelling in formability such as stretch flange formability.